Recently, studies are actively conducted to produce semiconductor devices, such as a blue LED, a white LED, and a blue-violet semiconductor laser, using a group 13 element nitride such as gallium nitride, and to apply the resulting semiconductor devices to various electronic apparatuses.
With the expansion of the uses of the white LED, there has been an increasing demand for further improving performance of an LED chip. For the LED chip, improving performance means achieving high efficiency and high luminance.
The HVPE methods are well known methods for producing a gallium-nitride self-supporting substrate. Of these, the DEEP method (Patent document 1 and Non-patent document 1) and the VAS method (Patent documents 2 and 3) are disclosed as methods for obtaining high-quality crystals.
In a currently commonly used structure, in which a GaN thin film (a light-emitting part) is formed on a sapphire substrate using the MOCVD method, a dislocation density of the light-emitting part is extremely high in a range of 108 to 109/cm2, thereby causing the non-radiative recombination of carriers in the dislocation regions. As such, it has been difficult to improve the light-emitting efficiency in such a structure.
On the other hand, the flux method is one of the liquid phase methods. For gallium nitride, the use of sodium metal as a flux can lower the temperature and pressure required for the crystal growth of gallium nitride to about 800° C. and several MPa, respectively. Specifically, nitrogen gas is dissolved in a mixed melt of sodium metal and gallium metal to bring gallium nitride into a supersaturated state, so that gallium nitride grows as crystals. In such liquid phase methods, dislocation hardly occurs as compared with the vapor phase methods, thereby allowing to obtain high-quality gallium nitride having a low dislocation density (Patent document 4).